sell » sail

When you support your C in C, that is loyalty, when to support your C in C unconditionally, that is stupidity, because he’ll sail you down the river for every political expediency going. (ivote2004.com forum, Apr. 13, 2004)

you wanted to sail me down the river cause something you thought you heard at a party when you were drunk. (StuntLife.com forum, May 10, 2004)

Also they are jealous of the fact that our party are not willing to sail us down the river to save them bad press unlike Sinn Fein. (Slugger O'Toole blog comment, Jan. 17, 2005)

Understandable enough, given the current opacity of the connection of “sell NP down the river” with the slave trade (and not, I gather, with the prison at Sing Sing) and the association of sailing with rivers, not to mention the phonology. But an eggcorn it is.